Heard It on the Radio

On August 27th, 1998, space probes positioned in different parts of the solar system registered a strange barrage of high-energy photons traveling through interplanetary space. Astrophysicists quickly identified the source of this fusillade as SGR1900+14, an object located some 20,000 light-years away that is thought to be a "magnetar"?a neutron star with an extraordinarily strong magnetic field. But even before its highly magnetic nature was discovered, observational astronomers had known that this star and several other "soft-gamma repeaters" located in or near our galaxy give off high-energy bursts from time to time. So the detection of gamma rays coming from this direction was not a complete surprise. What was astonishing was the intensity of the outpouring: The event sent detectors off scale in some of the spacecraft equipped to measure these emanations. Indeed, the radiation from this faraway star was strong enough to ionize parts of the earth's nighttime atmosphere almost as thoroughly as the sun does during the day.

In the week that followed, newspapers across the country ran stories describing the enormity of this celestial event. To convey some sense of magnitude, many of these accounts cited one astronomer's calculation that the energy released in five minutes by the magnetar would be sufficient to power all of civilization for billions of years to come. Yet investigators have recently determined that this initial estimate may have been off the mark: It now seems that the invisible flash of high-energy radiation (x rays and gamma rays) must have been an order of magnitude more intense than was first thought. This reassessment appears in an article by Umran S. Inan of Stanford University and five co-authors, published recently in the journal Geophysical Research Letters.

Inan is not an astrophysicist, but he and his colleagues at Stanford's Space, Telecommunications and Radioscience Laboratory had been collecting measurements from a network of very-low-frequency radio receivers. Their equipment is tuned into broadcasts from a small set of powerful transmitters that the U.S. Navy uses to communicate with its submarines at sea. Although the content of the military messages is undecipherable to them, investigators such as Inan and physicist Richard Dowden (of the company Low-Frequency Electromagnetic Research in Dunedin, New Zealand) monitor such transmissions routinely, in part because they convey quite a lot of information about the state of the earth's ionosphere.

This region of the atmosphere above about 50 kilometers' altitude is too high to be studied by balloons and too low to be monitored by satellites. But because the sun's rays split molecules in this zone into positively charged ions and negatively charged electrons, the ionosphere is more or less conductive and acts both as a screen and as a reflective ceiling for the radio waves that bounce back and forth between it and the ground as they travel around the globe.

When Inan and his colleagues looked carefully at their records of radio reception during the event, they saw immediately that transmissions passing through the hemisphere that had been briefly bathed in radiation from the magnetar were profoundly affected. The strength of radio signals propagating through the night sky dropped to levels as low as those normally seen during the day, when enhanced ionization from the sun's rays normally attenuates such transmissions.

What is more, in their record of signals traveling from Hawaii to Antarctica (where the background radio noise is quite low), Inan and his coworkers were able to discern a series of subtle oscillations in amplitude just after the initial blast of high-energy photons hit the earth. These variations match the very clear 5.16-second modulation registered by detectors on the Ulysses spacecraft, an oscillation that astronomers believe reveals the rotation rate of the distant source.

In preparing their scholarly publication, Inan and his associates tried to determine the exact nature of the ionospheric disturbance that the magnetar had briefly created. But when they used measurements from the Ulysses probe to specify the energy spectrum of the incoming rays, their computer model had difficulty reproducing the changes in radio reception. Indeed, the only way to match their observations was to assume that Ulysses had missed 90 percent of the action. So they hypothesized that for every high-energy photon registered by the spacecraft, there must have been nine undetected photons of somewhat lower energy.

Interestingly, the "Gamma Ray Burst" instrument Ulysses carries does, in fact, contain two solid-state detectors designed to measure just such low-energy photons, which are, technically, categorized as soft x rays. Kevin Hurley of the University of California, Berkeley?the investigator responsible for this instrument and one of Inan's co-authors?says that although the high-energy detectors continue to work well, the low-energy ones have aged badly since they were built, nearly two decades ago. They are now so noisy, Hurley explains, that they did not produce any useful measurements.

Although other space probes carry devices that are sensitive to photons in the hypothesized energy range, all are directional devices, and none was pointed at the right part of the sky when the burst took place. So these spaceborne instruments, too, were of little help in uncovering the soft x-ray component. In a sense, the earth's ionosphere provided the best detector available. "We measure it as well as anyone can measure it," quips Inan.

Indeed, the earth's ionosphere is so sensitive to incoming radiation, and radio propagation is so sensitive to changes in the ionosphere, that some casual radio listeners might well have observed effects of this distant neutron star even before scientists took note of it. Paul Harden, an engineer and amateur radio enthusiast who works at the National Radio Astronomy Observatory in Socorro, New Mexico, has been gathering anecdotal reports about changes in radio reception that day. In Harden's view, most of the accounts he received could be better ascribed to the effects of a geomagnetic storm raging at the time, but a few may reflect the gamma-ray burst. He describes, for example, the experience of a nurse in Seattle who was listening to her car radio on the way home from a late-night shift. (The burst took place at 3:22 a.m. Pacific time.) To her great surprise, the local FM station cut out and was replaced by one from Nebraska. According to Harden, it is not unusual for the increased ionization created by enhanced solar activity to cause such radio anomalies, but this woman's experience was truly peculiar because the weird events took place at night.

Inan and Dowden are both skeptical that the magnetar could have been responsible for changes in reception at the very high frequencies of the FM broadcast band. So perhaps this nurse's recollection was, in fact, more influenced by fatigue than by gamma radiation. But as Dowden confirms, there is no reason to think that some short-wave and AM broadcasts received that night would not have been affected. Reception of these signals over much of the Pacific hemisphere would presumably have faded out for a few minutes, all because the magnetic field of a distant neutron star released a vast amount of energy in a sudden burst some 20,000 years ago. Awesome, isn't it, what one can hear on the radio??David Schneider